Protectors for electric circuits



' Aug. 16,1966

' FIG. 2.

T. E. L. FITZ GERALD 3,267,240

PROTECTORS FOR ELECTRIC CIRCUITS Filed July 22, 1963 Patented August 16,1966 3,267,240 PROTECTORS FOR ELECTRIC CIRCUITS Thomas E. L. FitzGerald,St. Louis, Mo., assignor to Me- Graw-Edison Company, Elgin, 111., acorporation of Delaware I Filed July 22, 1963, Ser. No. 296,611

' 5 Claims. (Cl. 200131) a hurtful voltage surge or a hurtful currentsurge can not develop. It would be desirable to provide an electric fusethat could respond to a potentially hurtful current overload to promptlyopen the circuit without permitting a hurtful voltage surge or a hurtfulcurrent surge to develop. The present invention provides such anelectric fuse; and it is, therefore, an object of the present inventionto provide an electric fuse which can respond to a potentially hurtfulcurrent overload to promptly open the circuit in such a way that ahurtful voltage surge or a hurtful current surge can not develop.

The electric fuse provided by the present invention uses a bimetallicfusible element; and one part of that fusible element has higherresistance and a higher melting point than does another part of thatfusible element. The one part of that fusible element is preferably madeas a central core, and the other part of that fusible element ispreferably made as a coating for that core. The combined cross sectionsof the said one part and the said other part of that fusible elementwill be large enough to make the resistance of that fusible elementsmall enough to enable that fusible element to carry the rated currentof the fuse continuously without raising the temperature of the saidother part of that fusible element to the melting point of that otherpart. However, the combined cross sections of the said one part and thesaid other part of that fusible element will be small enough to make theresistance of that fusible element large enough to enable that fusibleelement to respond to a potentially hurtful current overload to raisethe temperature of the said other part of that fusible element to themelting point of that other part. As that other part of that fusibleelement reaches its melting point and starts to melt, its resistancewill increase; and the resulting increase in the overall resistance ofthe fusible element will tend to keep a hurtful current surge fromdeveloping. As portions of the said other part of that fusible elementfully melt, they will flow or draw away from the portions of the saidone part of the fusible element which they initially encased; and,thereupon, the overall resistance of the fusible element will againincrease. That further increase in the overall resistance of thatfusible element will additionally tend to keep a hurtful current surgefrom developing. The said portions of the said one part of the fusibleelement will then melt; and as those portions melt their resistance willincrease and will thus tend to keep a hurtful current surge fromdeveloping. The overall result is that the electric fuse provided by thepresent invention can respond to a potentially hurtful current overloadto open the circuit while keeping a hurtful current surge fromdeveloping. It is, therefore, an object of the present invention toprovide an electric fuse wherein the fusible element is bi-rnetallic,and one part of that fusible ele ment has a higher resistance and ahigher melting point than does another part of that fusible element.

As the fusible element of the electric fuse provided by the presentinvention fuses to open the circuit, it will experience the step-by-stepincrease in resistance described hereinbefore. However, once thatfusible element starts to fuse, it will rapidly open the circuit. Thisis due to the fact that the said one part of that fusible elementnormally carries only a small part of the overall current and is whollyincapable of carrying an overload current. As a result, when the saidother part of the fusible element fuses in response to a potentiallyhurtful current overload, the said one part of that fusible element willrespond to its subjection to that overload'to promptly fuse.

The said one part, of the fusible element of the electric fuse providedby the present invention, is preferably made from a metal which has avery high melting temperature and which has an even higher vaporizingtemperature. Specifically, that one part is preferably made from a metalwhich has a melting temperature and a vaporizing temperature that are,respectively, considerably higher than the melting temperature and thevaporizing temperature of platinum. Where that one part is made fromsuch metal, the vapors which are generated as that one part fuses willbe at, or close to, the thermal ionization temperature of those vapors;and those vapors will promptly establish an arc. That are will providesuch a low resistance path through the fuse that a sharp rise in thevoltage across that fuse cannot occur. This action is in contrast to thecircuit opening action of most fuses, wherein the temperatures of thevapors of the fusible elements are initially well below the thermalionization temperatures of those vapors and wherein the voltage acrossthe fuse must increase until those vapors can be ionized sufficiently toenable arcs to form. The voltage increases across those fuses areusually rapid and substantial; and those voltage increases producevoltage surges which could injure some electric devices. The presentinvention obviates all such voltage increases by incorporating into thefusible element of the fuse thereof a metal which has a vaporizingtemperature at, or close to, the thermal ionization temperature of thatmetal.

An electric fuse with a fusible element consisting of a metal that had avaporizing temperature at, or close to, the thermal ionizationtemperature of that metal would be desirable because it would promptlyestablish an are when it fused. However, such a fuse would beundesirable because the fusible element thereof would have an undulyhigh resistivity. The present invention attains the benefits of afusible element made from a metal which has a vaporizing temperature at,or close to, the thermal ionization temperature of that metal and yetavoids an unduly high resistivity for the fusible element of the fusethereof by providing a bi-metallic fusible element for that fuse. Thesaid one part of that fusible element will be the last to fuse, and thefusing temperature of the metal thereof will be so close to the thermalionization of that metal that the said one part will facilitate promptarc formation, and will thus obviate a potentially hurtful voltagesurge. The metal of the other part of that fusible element will have aresistivity which is smaller than the resistivity of the metal of thesaid one part of that fusible element; and hence that other part of thefusible element will reduce the overall resistivity of the fusibleelement to a practical value. It is, therefore, an object of the presentinvention to provide an electric fuse which has a bi-metallic fusibleelement with one part thereof made from a metal that has a vaporizingtemperature at, or close to, the thermal ionization temperature of thatmetal .of those links and the said support.

3 and with another part thereof made from a metal having a resistivitysmaller than the resistivity of the metal of the said one part of thatfusible element.

To enable theelectric fuse, provided by the present invention, to keeppotentially hurtful low overloads from injuring the electric devices inthe circuit protected by that fuse, that fuse' must be able to open thatcircuit promptly. The present invention makes it possible for that fuseto open the circuit promptly by sub-dividing the fusible element of thatfuse into a number of short, parallel-connected links and by mountingthose links on a massive support of dielectric material which has agroove in the exterior thereof. The sub-dividing of the fusible elementinto va number of short, parallel-connected links enables that fusibleelement to have a smaller mass than that fusible element would have ifit were not subdivided; and that smaller mass enables the fusibleelement to fuse more rapidly than it could if it were not sub-divided.The massive support of dielectric material absorbs substantialquantities of heat from those portions of the links which are tightlyheld in intimate engagement with it, and thus keeps. the greaterportions of the lengths of those links relatively cool. The groove inthat support enables those portions of the links which span it to becomequite hot; .and the temperatures of those portions of those links will,whenever the fuse is carrying its rated current, be close to thetemperature at which those links can start .opening the circuit. Theoverall result is that while the greatest portion of the lengths of thelinks will be kept relatively cool-and thus can help extinguish any arcsthat form as the fuse opensthe portions of the links which span thegroove. can respond to the rated current of the fuse to operate attemperatures close to the temperature at which those links can startopening the circuit and can respond to potentially hurtful overloads torapidly reach the temperatures at which they can start opening thecircuit. It is, therefore, an object of the present invention to providea fuse wherein the fusible element is subdivided into a number of short,parallel-connected links that are mounted on a massive support ofdielectric material which has a groove in the exterior thereof.

To remain relatively cool, the portions of the links of the fuse whichengage the massive support of dielectric material must remain in tightand intimate engagement with that support. If those links weremono-metallic in nature, the portions thereof which span the groove inthe support and which respond to the flow of the rated current throughthe fuse to operate at temperatures close to the temperature at whichthose links can start opening the circuit would experience such alessening of the tensile strength thereof that a tight and intimateengagement could not be maintained between the remaining portionsHowever, because the links of the present invention are bi-metallic innature and because one of the metals of those links has a tensilestrength which is substantially undiminished at temperatures up to andincluding the temperature at which the other metal of those links meltsand starts the opening of the circuit, those links are able to remain intight and intimate engagement with that support. It is, therefore, anobject of the present invention to provide a fuse wherein the fusibleelement remains in tight and intimate engagement with a massive supporteven though some portions of that fusible element respond to the flow ofthe rated current through the fuse to operate at temperatures close tothe temperature at which those portions can start opening the circuit.

The electric fuse provided by the present invention has connectors thatare massive relative to the links of that fuse, and those connectorsdirectly engage those links. As a result, those connectors help themassive support of dielectric material keep the greatest portions of thelengths of those links relatively cool. It is, therefore, an object ofthe present invention to provide an electric fuse with a massive supportof dielectric material and with relatively massive connectors whichdirectly engage the links of that fuse.

Other and further objects and advantages of the present invention shouldbecome apparent from an examination of the drawing and accompanyingdescription.

In the drawing and accompanying description a preferred embodiment ofthe present invention is shown and described, but it is to be understoodthat the drawing and accompanying description are for the purpose ofillustration only and do not limit the invention and that the inventionwill be defined by the appended claims.

In the drawing,

FIG. 1 is a front elevational view of a support .of dielectric materialfor one form .of electric fuse that is made in accordance with theprinciples and teachings of the present invention,

FIG. 2 is an elevational view of the left-hand end of the support ofFIG. 1,

FIG. 3 is a front elevational view of the support of FIG. 1 after it hashad connectors secured thereto and has had a fusible element secured tothose connectors,

.FIG. 4 is an elevational view of the left-hand end of the support, thefusible element and the connectors of FIG. 3,

FIG. 5 is a vertical section through a preferred form of electric fusewhich includes the support, the connectors and the fusible element ofFIGS. 3 and 4,

FIG. 6 is a sectional view through the electric fuse of FIG. 5, and itis taken along the plane indicated by the line 6-6 in FIG. 5,

FIG. 7 is an elevational view of the left-hand end of one of theterminals for the electric fuse of FIG. 5,

FIG. 8 is a sectional view through the terminal of FIG. 7, and it is.taken along the plane indicated by the line 88 in FIG. 7, and

FIG. 9 is a sectional view, on a larger scale, through the fusibleelement of FIGS 3-5.

Referring to the drawing in detail, the numeral 10 denotes a support ofdielectric material, and that dielectric material should besubstantially unaffected by heat or by electric ..arcs,.and it should bedimensionally stable. One material that has been found to be very usefulis steatite.

The support 10 is generally cylindrical, and it has a generally circularcross section. However, that support has semi-cylindrical, elongatedgrooves 12, 14, 16 and 18 formed in thesurface thereof. Those groovesare parallel to the geometric axis of the support 10; and the grooves 12and 16 are disposed at the ends of one diameter of that support whilethe grooves 14 and 18 are disposed at the ends of another diameter ofthat support. In the preferred form of the present invention, the widthof each groove is less than one-thirteenth of the circumference of thesupport 10. In that preferred form, the said dielectric support 10 isabout one and thirty-six thousandths of an inch long and is about onehundred and eighty-five thousandths of an inch in diameter. The widthand depth of each groove is about forty-three thousandths of an inch.

The numeral 20 denotes a J-shaped connector which is preferably madefrom a length of stifi wire of copper or the like. The diameter of thewire used in making the connector 20 is preferably just slightly lessthan the diameter of the grooves 12 and 16; and, in the preferred formof the present invention, is about forty thousandths of an inch.Furthermore, the center-to-center distance of the arms of the J-shapedconnector 20 is preferably equal .to the center-to-center distance ofthe grooves 12 and 16. As a result, the long arm of the connector 20 canbe lodged within the groove 12 while the short :arm of that connectorcan be lodged within the groove 16. The numeral 22 denotes a secondconnector which is substantially identical to the connector 20. However,the short arm of the connector 22 is lodged within the groove 12 Whilethe long arm of the connector 22 is lodged within the groove 16. Theconnectors and 22 are dimensioned so the long arm of the connector 20 isspaced an appreciable distance from the short arm of the connector 22and so the long arm of the connector 22 is spaced an appreciabledistance from the short arm of the connector 20. Those spacings are longenough to prevent any leakage of current between the connectors 20 and22.

The numeral 24 generally denotes a fusible element which has a centralcore 23 and a coating 25. That central core preferably is an elongatedwire which is made from a metal that has a high melting point and ahigher vaporizing temperature and that has a high tensile strength.While different metals could be used in making the core 23, tungsten hasbeen found to be very useful. The coating has a melting pointconsiderably below the melting point of the metal of the core 23; andthat coating is preferably made from one or more metals that canbe'electroplated onto that core but which will not tend to alloy withthe metal of that core. Where the coating 25 includes two metals, thosemetals will preferably be formed as separate layers or sheaths. Wherethe coating 25 has just one metal therein, that metal can be copper.Where that coating has two metals therein, one can be copper and theother can be silver.

For clarity of illustration, the fusible element 24 has been shown muchlarger than it is in actual practice. In the said preferred embodimentof the present invention, the core 23 for the fusible elements 24 of onethrough five ampere fuses is a number fifty tungsten wire; and the core23 for the fusible elements 24 of seven through nine ampere and fifteenampere fuses is a number fortyeight tungsten wire. The fusible element24 for the one ampere fuse has just enough copper electroplated onto thecore23 to reduce the resistance of that fusible element to aboutthirty-five ohms per foot. The fusible element 24 for the two amperefuse has just enough copper electroplated onto the core 23 to reduce theresistance of that fusible element to about eighteen ohms per foot. Thefusible element 24 for the three ampere fuse has just enough copperelectroplated onto the core 23 to reduce the resistance of that fusibleelement to about fourteen ohms per foot. The fusible element 24 for thefour ampere fuse or the five ampere fuse has just enough copperelectroplated onto the core 23 to reduce the resistance of that fusibleelement to about ten and one-half ohms per foot. The fusible element 24for the seven ampere fuse or the eight ampere fuse has just enoughcopper electroplated onto the core 23 to reduce the resistance of thatfusible element to about seven and one-quarter ohms per foot. Thefusible element 24 for the nine ampere fuse has just enough copperelectroplated onto the core 23 to reduce the resistance of that fusibleelement to about six and one-half ohms per foot. The fusible element 24for the fifteen ampere fuse has just enough copper electroplated ontothe core 23 to reduce the resistance of that fusible element to aboutseven and one-quarter ohms per foot and then has enough silverelectroplated onto the copper to reduce the overall resistance of thatfusible element to about two and one-half ohms per foot. The fusibleelements 24 for larger amperage fuses will use larger diameter cores 23and will use thicker coatings 25. As a general rule, where the fusibleelement 24 consists of a tungsten core 23 and a copper coating 25, theweight of the coating 25 should be from about eighty to about onehundred percent of the weight of the core 23. Where the fusible element24 consists of a tungsten core 23 and a coating 25 which includes acopper layer and an external silver layer, the weight of the copperlayer should be from about eighty to about one hundred percent of theweight of the core 23 and the weight of the silver layer should be fromabout one hundred and seventy to about one hundred and fifty percent ofthe weight of that core. These weight relationships are not practicallyattainable in electric fuses with ratings of. less than ten amperes,

6 because tungsten wire of the requisite minute diameter is notcommercially available. However, those weight relationships can andshould be used in larger fuses. In each fuse, regardless of its ampererating, the support 10 should be made large enough to be massiverelative to the fusible element.

The fusible element 24 is arranged to act as a plurality of short,parallel-connected links. Specifically, that fusible element is wound inhelical fashion onto the support 10; and the various turns of thatfusible element engage and are bonded to the long arms of the connectors20 and 22. A mass 26 of solder bonds the turns of that fusible elementto the connector 20, and a mass 28 of solder bonds those turns to theconnector 22. The fusible element 24 is wound tightly onto the support10, and the various turns thereof are held in intimate engagement withthe periphery of that support as the masses 26 and 28 are used to bondthose turns to the long arms of connectors 20 and 22. The intimateengagement between fusible element 24 and support 10 is importantbecause it enables substantial amounts of the heat generated by thatfusible element to be absorbed by that support.

While the greatest portions of the length of each turn offusible'element 24 are in intimate engagement with the periphery ofsupport 10, the portions of those turns which span the grooves 14 and 18are not in engagement with the periphery of that support. As a result,those latter portions can become quite hot; and, preferably, thetemperatures of those latter portions will, whenever the rated currentflows through fusible element 24, be close to the melting temperature ofcoating 25.

The number of turns of the fusible element 24 will vary with the ampererating of the fuse. For example, in the preferred form of the presentinvention, the one ampere fuse has one and one-half turns, the twoampere fuse and the three ampere fuse each has two turns, the fourampere fuse has three turns, the five ampere fuse has three and one-halfturns, the seven ampere fuse has three and onehalf turns, the eightampere fuse has four and one-half turns, the nine ampere fuse has fourturns, and the fifteen ampere fuse has three and one-half turns. Byappropriate selection of the number of turns, and by appropriateselection of the weights of the cores 23 and the coatings 25, it ispossible to make fuses of any desired ampere rating.

The support 10, the connectors 20 and 22, the fusible element 24, andthe masses of solder 26 and 28 constitute a sub-assembly which is sturdyand rugged. That sub-assembly can readily be handled as a unit.

The numeral 32 denotes a generally cup-shaped terminal which has arecessed end wall. An elongated rectangular slot 34 is formed in therecessed end wall of the terminal 32, and that slot is dimensioned toaccommodate the closed end of the J-shaped connector 20-all as shown byFIG. 5. The numeral 36 denotes a cup-shaped terminal which has arecessed end wall; and an elongated rectangular slot 38 is formed inthat recessed end wall. The slot 38 is dimensioned to accommodate theclosed end of the J-shaped connector 22, all as shown by FIG. 5.

The numeral 30 denotes a casing of dielectric material. That casing hasan inner diameter which is larger than the diameter of the support 10;and it has an outer diameter which is just slightly smaller than theinner diameters of the terminals 32 and 36.

In assembling the electric fuse provided by the present invention, theconnectors 20 and 22 are set within the grooves 12 and 16 in the support10; and thereafter the fusible element 24 is wound into intimateengagement with the support 10 and with the connectors 20 and 22. Thatfusible element is then maintained in intimate engagement with thatsupport and with those connectors while the masses 26 and 28 of solderare used to bond that fusible element to those connectors. The resultingsub-assembly can then be telescoped into the casing 30.

'to generate heat.

The terminal 32 can have the slot 34 therein alined with 'the closed endof the connector 20, and then the open end of that terminal can betelescoped over the left-hand end of the casing 30. That open'end can,at such time, be crimped to fixedly secure it to the casing 30.Thereupon the terminal 36 can have the slot 38 therein alined with theclosed end of the connector 22, and can have the open end thereoftelescoped over the right-hand end of the casing 30. That open end can,at that time, be crimped into permanent engagement with the casing 30.Solder 40 can be used to bond the J-shaped connector 20 to the terminal32, and further solder 42 can be used to bond the J-shaped connector 22to the terminal 36. The terminals 32 and 36 are dimensioned to fit intostandard fuse clips.

In the normal operation of the electric fuse provided by the presentinvention, current will flow from terminal 32 via solder 40, connector20, solder mass 26, the short,

parallel-connected links which are defined by the fusible element 24 andare shown by solid lines in FIG. 3 and the short, parallel-connectedlinks which are defined by the fusible element 24 and are shown bydotted lines in FIG. 3, the solder mass 28, connector 22, and solder 42to the terminal 36. The flow of current through the links defined by thefusible element 24 Will cause those links Because the greatest portionsof the lengths of those links are held tightly in intimate engagementwith the periphery of the support or with the connectors 20 and 22,those portions will remain relatively cool. However, those portions ofthe lengths of the links which span the grooves 14 and 18 will becomequite hot. In fact, when the fuse of FIGS. 5 and 6 is carrying its ratedcurrent, the temperatures of the portions'of the links which span thegrooves 14 and 18 will be close to the melting point of the coating 25.However, as long as the current passing through that fuse does notexceed the rated current of that fuse, the links defined by the'fusibleelement 24 will remain intact.

In the event the current flowing through the fuse of FIGS. 5 and 6exceeds the rated current by a predetermined amountand that amount canbe as low as ten percent of the rated currentthe temperatures of thoseportions of the links which span the grooves 14 and 18 will reach themelting point of the coating 25. Thereupon, those portions of thecoating 25 will melt, and will flow or draw away from those portionsofthe core 23 which they initially encased. As those portions of thecoating 25 melt, the resistances thereof will increase; and this meansthat the overall resistances of the various links will increase. This isdesirable because it will tend to keep a hurtful current surge fromdeveloping. Also, as the said portions of the coating 25 melt, the cores23 of the'links will have to carry higher percentages of the current;and those cores will get hotter because of that fact. As the saidportions of the coating 25 flow or draw away from the portions of thecores 23 which they initially encased, the overall resistances of thevarious links will again increase. This is desirable because it willtend to keep a hurtful current surge from developing. Also, as the saidportions of the coating 25 draw or flow away, the said portions of thecores 23 will be subjected to the full current overload; and thoseportions of those cores will promptly melt. As those portions of thecores 23 melt, the overall resistances of the links will increase onceagain. This is desirable because it will tend to keep a current surgefrom developing. Very promptly, the said portions of the cores 23 willvaporize; and, immediately, arcs will form, because the temperatures ofthe vapors of those portions of those cores will be close to the thermalionization temperatures of the metal of those cores. Those arcs willavoid an abrupt rise in the voltage across the fuse of FIGS. 5 and 6,and will thus tend to keep a hurtful voltage surge from developing. Thearcs will be quickly extinguished because the support 10, and those '8portions of the fusible element 24 which intimately engage that support,will be cool and can quickly de-ionize those arcs. The overall result isthat the fuse of FIGS. 5 and 6 can respond to potentially hurtfulcurrent overloads and open the circuit without permitting hurtfulcurrent surges or hurtful voltage surges to develop.

In providing progresive increases in the overall resistance of thefusible element 24, the coating 25 and the core 23 not only tend to keephurtful current surges from developing, but they also tend to keephurtful voltage surges from developing. Specifically, those progressiveincreases in the overall resistance of the fusible element 24 limit therate of change of the current in the circuit, and thus reduce the valueof the voltage which can be induced in that circuit. The overall resultis that the fuse provided by the present invention can respond topotentially hurtful current overloads to open the circuit withoutpermitting hurtful current surges or hurtful voltage surges to develop.

Where desired, arc-extinguishing filler material, not shown, can bedisposed in the space between the support 10 and the interior of thecasing 30. That filler material will embed the portions of the linkswhich span the grooves 14 and 18, and will help extinguish the arcs thatform as those portions fuse. In most sizes of fuses provided by thepresent invention, filler material will be wholly unnecessary because ofthe relative massiveness of the support 10. Even in the thirty amperefuse provided by the present invention, the over-all diameter of thecopper and silver coated fusible element 24 is only twenty-sixten-thousandths of an inch; and the arcs which form when portions ofthat fusible element fues are quickly de-ionized by the relatively coolsupport 10 and by those portions of the links which are tightly held inintimate engagement with that support.

While tungsten is particularly useful as the metal for the core 23,because of its high vaporizing temperature, its workability, and itscommercial availability, other metals could be used. If they could becommercially obtained as wire of the required diameter, metals such asiridium, osmium, molybdenum, rhenium, tantalum, titanium, yttrium andzirconium could be used in making the core 23.

Whereas the drawing and accompanying description have shown anddescribed one preferred embodiment of the present invention, it shouldbe apparent to those skilled in the art that various changes may be madein the form of the invention without affecting the scope thereof.

What I claim is:

1. An electric fuse which comprises:

(a) terminals that are 'connectable into an electric circuit,

(b) a fusible element electrically connected to said terminals toconduct current between said terminals,

(0) said fusible element responding to overloads of predeterminedmagnitude and duration to fuse,

(d) a massive support of dielectric material that supports said fusibleelement and that absorbs heat from said fusible element,

(e) connectors connecting said fusible element to said terminals,

(f) said connectors being generally I-shaped,

(g) said support having a groove therein to accommodate .the long arm ofone of said connectors and to accommodate the short arm of a second ofsaid connectors,

(h) said support having a second groove therein to accommodate the longarm of said second connector and to accommodate the short arm of saidone connector,

(i) said support having a third groove intermediate the first said andsaid second grooves,

(j) said fusible element spanning said third groove.

2. An electric fuse which comprises:

(a) terminals that are connectable into an electric circuit,

(b) a fusible element electrically connected to said terminals toconduct current between said terminals,

(c) said fusible element responding to overloads of predeterminedmagnitude and duration to fuse,

(d) a massive support of dielectric material that supports said fusibleelement and that absorbs heat from said fusible element,

(e) connectors connecting said fusible element to said terminals, (f)said support having a groove therein whichextends transversely of saidfusible element and which accommodates one of said connectors,

(g) said support having a second groove therein which extendstransversely of said fusible element and which accommodates a secondconnector,

(h) said support having a third groove therein which extendstransversely of said fusible element and which is intermediate the firstsaid and said second grooves,

(i) said fusible element having a portion thereof spanning said thirdgroove and having elongated portions that are contiguous with saidspanning portion and that intimately engage said support,

(j) said support absorbing substantial amounts of heat from saidcontiguous portions of said fusible element,

(k) said spanning portion of said fusible element having much less heatabsorbed from it than do said contiguous portions of said fusibleelement,

(1) whereby said spanning portion of said fusible element is hotter thansaid contiguous portions of said fusible element,

(m) said fusible element responding to overloads of predeterminedmagnitude and duration to cause said spanning portion thereof topromptly fuse.

3. An electric fuse which comprises:

(a) terminals that are connectable into an electric circuit,

(b) a fusible element electrically connected to said terminals toconduct current between said terminals,

(c) said fusible element responding to overloads of predeterminedmagnitude and duration to fuse,

(d) a massive support of dielectric material that supports said fusibleelement and that absorbs heat from said fusible element,

(e) said support defining a space that is spanned by a portion of saidfusible element,

(f) the portions of said fusible element which are contiguous to saidspanning portion of said fusible element being elongated and being heldin intimate engagement with the surface of said support,

(g) whereby said contiguous portions of said fusible element will bekept relatively cool and will be resistant to fusing,

(h) said spanning portion of said fusible element having much less heatabsorbed from it than is absorbed from said contiguous portions of saidfusible element,

(i) whereby said spanning portion of said fusible element is hotter thansaid contiguous portions of said fusible element,

(j) said fusible element being dimensioned so the temperature of saidspanning portion thereof will be close to the fusing temperature thereofwhen rated current flows through said fusible element.

4. An electric fuse which comprises:

(a) terminals that are connectable into an electric circuit,

(b) a fusible element electrically connected to said terminals toconduct current between said terminals,

(c) said fusible element having a plurality of sections that areconnected in parallel with each other,

(d) said fusible element responding to overloads of predeterminedmagnitude and duration to fuse,

(e) a massive support of dielectric material that supports said sectionsof said fusible element and that absorbs heat from said sections of saidfusible element,

(f) said support defining a space that is spanned by a portion of eachof said sections of said fusible element,

(g) the portions of each section of said fusible element which arecontiguous to said spanning portion of said section of said fusibleelement being elongated and being held in intimate engagement with thesurface of said support,

(h) whereby said contiguous portions of said sections of said fusibleelement will be kept relatively cool and will be resistant to fusing,

(i) said spanning portions of said sections of said fusible elementhaving much less heat absorbed from them than is absorbed from saidcontiguous portions of said sections of said fusible element,

(j) whereby said spanning portions of said sections of said fusibleelement are hotter than said contiguous portions of said sections ofsaid fusible element,

(k) said sections of said fusible element being dimensioned so thetemperature of the spanning portion of each of said sections of saidfusible element will be close to the fusing temperature thereof whenrated current flows through said fusible element,

(1) whereby said spanning portions of said sections of said fusibleelement will promptly fuse on very low overloads.

5. An electric fuse which comprises:

(a) terminals that are connectable into an electric circuit,

(b) a fusible element electrically connected to said terminals toconduct current between said terminals,

(c) said fusible element responding to overloads of predeterminedmagnitude and duration to fuse,

(d) a massive support of dielectric material that supports said fusibleelement and that absorbs heat from said fusible element,

(e) said support defining a space that is spanned by a portion of saidfusible element,

(f) the portions of said fusible element which are contiguous to saidspanning portion of said fusible element being elongated and being heldin intimate engagement with the surface of said support,

(g) whereby said contiguous portions of said fusible element will bekept relatively cool and will be resistant to fusing,

(h) said spanning portion of said fusible element constituting only aminor portion of the length of said fusible element,

(i) said contiguous portions of said fusible conductor constituting themajor portion of the length of said fusible element,

(j) whereby the major portion of the length of said fusible conductor iskept relatively cool and resistant to fusing,

(k) said spanning portion of said fusible element having much less heatabsorbed from it than is absorbed from said contiguous portions of saidfusible element,

(1) whereby said spanning portion of said fusible element is hotter thansaid contiguous portions of said fusible element,

(In) said spanning portion of said fusible element having, because ofthe elevated temperature thereof, a greater resistivity than theresistivity of any of said contiguous portions of said fusible element,

(11) said fusible element being dimensioned so the temperature of saidspanning portion thereof will be close to the fusing temperature thereofwhen rated current flows through said fusible element.

(References on following page) 1:1 12 References Cited by the ExaminerFOREIGN PATENTS UNITED STATES PATENTS 267,828 5/1913 y;

470,014 3/1892 Scott et al 200 13s 919,696 4/1909 Cook 200131 51,036,510 8/1912 Murray 200-135 OTHER REFERENCES ,0 ,34 I 3 Hop 200135Encyclopedia of Chemical Technology, vol. 14, Inter- 1,335,229 3/1-920Eustice 200132 science Encyclopedia, Inc. of New York, 1955, pp.2,576,405 11/1951 McAlister 200131 353-372. 1 2,828,390 3/1958-McAlister 200-131 10 2 72 031 2 19 COX BERNARD A. GILHEANY, PrimaryExaminer.

3,069,520 12/1962 Cameron 200-120 H. B. GILSON, Assistant Examiner.

1. AN ELECTRIC FUSE WHICH COMPRISES: (A) TERMINALS THAT ARE CONNECTABLEINTO AN ELECTRIC CIRCUIT, (B) A FUSIBLE ELEMENT ELECTRICALLY CONNECTEDTO SAID TERMINALS TO CONDUCT CURRENT BETWEEN SAID TERMINALS, (C) SAIDFUSIBLE ELEMENT RESPONDING TO OVERLOADS OF PREDETERMINED MAGNITUDE ANDDURATION TO FUSE, (D) A MASSIVE SUPPORT OF DIELECTRIC MATERIAL THATSUPPORTS SAID FUSIBLE ELEMENT AND THAT ABSORBS HEAT FROM SAID FUSIBLEELEMENT, (E) CONNECTORS CONNECTING SAID FUSIBLE ELEMENT TO SAIDTERMINALS, (F) SAID CONNECTORS BEING GENERALLY J-SHAPED, (G) SAIDSUPPORT HAVING A GROOVE THEREIN TO ACCOMMODATE THE LONG ARM OF ONE OFSAID CONNECTORS AND TO ACCOMMODATE THE SHORT ARM OF A SECOND OF SAIDCONNECTORS, (H) SAID SUPPORT HAVING A SECOND GROOVE THEREIN TOACCOMMODATE THE LONG ARM OF SAID SECOND CONNECTOR AND TO ACCOMMODATE THESHORT ARM OF SAID ONE CONNECTOR, (I) SAID SUPPORT HAVING A THIRD GROOVEINTERMEDIATE THE FIRST SAID AND SAID SECOND GROOVES, (J) SAID FUSIBLEELEMENT SPANNING SAID THIRD GROOVE.